Variable delay line



:3 Sheets-Sheet 1 July 7, 1970 z. G. LYON VARIABLE DELAY LINE Filed Aug. 7. 1968 INVENTOR ZE/VO q. (.Yo/v .BY;

ATTORNEY 2 Sheets-Sheet z INVENTOR z-o q. LYON BY :2 r E t ATTORNEY July 7, 1970 z. G. LYON VARIABLE DELAY LINE Filed Aug. 7. 1968 H @V 2 m @m ll w. mav m nan A A J M ummU United States Patent Oflice 3,519,963 Patented July 7, 1970 3,519,963 VARIABLE DELAY LINE Zeno G. Lyon, Scotch Plains, N.J., assignor to International Telephone and Telegraph Corporation, Nutley,

N..I., acorporation of Delaware Filed Aug. 7, 1968, Ser.'No. 750,798

Int. Cl. H03h 7/36 US. Cl. 333-29. 10 Claims ABSTRACT OF THE DISCLOSURE A compact continuously variabledelay line is provided which has a constant characteristic impedance over its range of delay. The arrangement maintains a constant L/ C ratio by one adjustment which causes a simultaneous change in L and C.

BACKGROUND OF THE INVENTION In general this invention relates to delay lines, and more particularly to a continuously variable delay line using a coaxial helical line In the prior art, compact coaxial helical delay lines are not provided which are capable of being continuously variable over their range of operation and which maintain a constant characteristic impedance over this range with a low insertion loss. Many require the use of ferrite material or some variable material with its accompanying restriction in frequency. Other arrangements which utilize a helix do not have provision for removing the effects of the length of the helix-beyond the movable adjusting contact from elfecting the transmission line. The arrangement presented, which overcomes the limitation found in the prior art, maintains a constant L/C ratio by one adjustment and incorporates a low loss dielectric material in a manner not heretofore utilized in the helical delay line art.

SUMMARY OF THE INVENTION It is therefore an object of this invention to provide a variable delay line having a constant characteristic impedance over its range of delay.

Accordingly, there is provided a variable delay line including an outer cylinder adapted to couple to a transmission line, a helix positioned within said cylinder and coupled to said transmission line, a rod positioned to be selectively inserted with said helix in order to change the inductive characteristic of said helix, and means within said outer cylinder which is coupled to and movable with said rod to change the capacitive characteristic of said delay line in the same proportion as the inductive change due to the rods movement, thereby maintaining a constant characteristic impedance of said delay line over its range of delay.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying description will best be understood by reference to the accompanying drawings in which:

FIG. 1 illustrates an embodiment of the invention shown in a maximum delay position;

FIG. 2 shows the embodiment of FIG. 1 in a minimum delay position; and

FIG. 3 shows another embodiment of the invention in an intermediate position of delay.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A small delay line can be built by using a coaxial helical line. The characteristic impedance of such a line is represented by Z where L is the inductance and C is the capacitance of the helical line:

and the phase velocity is given by:

Since large values of LC will reduce the velocity, having a given Z and velocity, the two equations can be solved simultaneously to yield values of L and C.

The delay line length can be made continuously variable by utilizing materials of one sort or another to change the L/C ratio. However, if one desires that Z remain constant, it is necessary to change L and C simultaneously keeping the ratio constant.

One arrangement for accomplishing this is illustrated in the delay line of FIGS. 1 and 2, wherein an outer cylinder 10 has means 11 for coupling a signal in from a transmission line 12 and out again to the transmission line. Within the outer cylinder 10 is located an inner cylinder 13 having a slot 14 which permits the coupling from the transmission line to a helix 15. The slot permits the movement of the inner cylinder 13 from the position shown in FIG. 1 to the position shown in FIG. 2. Within the inner cylinder is located a rod comprising a low loss dielectric portion 16 and a high conductivity portion 17, and the cylinder 13 and rod being coupled for joint movement by insulated disk 18.

With the inner cylinder 13 completely inserted as shown in FIG. 1, a maximum delay will occur with L equal to L and C equal to C The phase velocity is then represented by:

1/ 1 1 The characteristic impedance is then represented by:

E Z 01 a When the inner cylinder is withdrawn as shown in FIG. 2 then the velocities are equated as:

The metallic portion of the rod reduces the inductive characteristic by the factor 1/ k and the iimer diameter 19 of the outer cylinder 10 is chosen to reduce the capacitive characteristic by the same 1/ k. Therefore the new impedance is represented by:

l/k L1 Z02 l/k 1 and the characteristic impedance remains constant.

As shown in FIG. 1, the maximum delay position provides a maximum delay to the transmission line by virtue presenting a high value of L to the line because of the dielectric rod being inserted within the helix, and also a high value of C because the inner diameter 29 of the inner cylinder 13 is small. As the inner cylinder and rod are moved simultaneously toward the right, the minimum delay position is reached as shown in FIG. 2. In this position the value of L is smaller because of the slug which is now inserted within the axis of the helix and a smaller value of C exists because of the larger inner diameter 19 of the outer cylinder 10. At intermediate points it is obvious that there is proportional changes in phase velocity, however, the characteristic impedance Z is constant. The transition at point 21 is sharp, but this transition can be made :more gradual by tapering the metal rod if this sharp variation cant be tolerated.

Large delays can be achieved with moderately small physical sizes because of the small values of V that can be obtained. The limiting size depends on the allowable insertion loss. In this embodiment the delay line provides a larger OD and a shorter length than the embodiment shown in FIG. 3 which represents an arrangement utilizing a smaller OD and a longer length.

Referring now to FIG. 3, the other embodiment comprises an outer cylinder 30 and a dielectric inner cylinder 31. The outer cylinder includes input means 32 at one end to couple to a transmission line and output means 33 to again re-couple the signal to the transmission line after it has passed through the helix 34. Helix 34 is dimensioned to pass Within the inner cylinder 31 and at the exit 35 is fixed a contact 36 which contacts a highly conductive rod 37 which is rotatable as indicated by arrows 38 and movable to a position of maximum delay or minimum delay as indicated by arrows 39 and 40 respectively. The rod is coupled to move with the inner cylinder and has at least one rotatable contact 41 which rotates with the rod to contact inner peripherial portions of the helix. The portion of helix 34 which is located between contacts 36 and 41 are shorted and do not affect the delay. The efiective delay length of the line is determined by the length 42 which is not shorted and which is surrounded by the dielectric inner cylinder 31 so that the combination of the air core helix portion 42 and the width 43 of the inner cylinder determines the value of L and C which change in the same proportion according to the movement of the rod and cylinder. in order to compensate for the transmission line impedance from the point of input coupling 32 to the beginning 44 of the helix, a coaxial cylinder 45 is provided for proper line impedance matching since the space or length 46 in [front of the helix must equal the length 47 of the helix. The cylinder 31 and rod 37 may then move to the position of maximum delay which is at the right of the figure, and to a position of minimum delay which is at the left of the figure. The maximum delay occurs at the position in which no turns are shorted, the helix comprises an air core, and the dielectric cylinder completely surrounds the helix along its axis. This provides a value of C which is in proportion to the value of L.

The choice of embodiment will depend on the arrangement in which it is to be used. If a larger diameter and shorter space is available, then the embodiment of FIGS. 1 and 2 would be preferable, and if a smaller diameter and longer space is available, then the embodiment of FIG. 3 would be preferable.

Accordingly, there is provided a variable delay line which has a constant characteristic impedance, wherein an outer cylinder encloses a helix and an inner cylinder which is selectively movable along the axial length of the helix to effect the capacitive characteristic of the delay line; and a rod which is coupled to and movable with the inner/cylinder affects the inductive characteristic of said helix, such that L and C increase or decrease in the same direction and maintain a constant characteristic impedance of the delay line.

I claim:

1. A variable delay line having a constant characteristic impedance comprising:

an outer cylinder adapted for coupling to a transmission line;

a helix positioned within said cylinder and adapted to be coupled to said transmission line;

a rod positioned to be selectively inserted within said helix to change the inductive characteristic of said helix; and I I means coupled to and movable with said rod to change the capacitive characteristic of said delay line in the same proportion as the change of inductive characteristic due to its movement, thereby maintaining the constant characteristic impedancevof said delay line.

2. A variable delay line according to claim 1 including an inner cylinder selectively movable within said outer cylinder so as to surround said helix along its axial length, whereby the portion of said inner cylinder which surrounds said helix effects the capacitance characteristic of said delay line. I

3. A delay line according to claim 2 wherein said rod is fixed within said inner cylinder and comprises a conduc tive portion and a dielectric portion, such that when said dielectric portion is inserted within said helix, said inner cyilnder completely surrounds the axial length of said helix, and said variable delay line presents a maximum delay to the transmission line. 4. A variable delay line according to claim 3 wherein when said conductive rod is completely inserted within said helix and said inner cylinder is completely withdrawn from said helix,said variable delay line presents a minimum delay to the transmission line and the same characteristic impedance as the position of maximum delay.

5. A variable delay line according to claim 2 wherein said inner cylinder is a dielectric cylinder movable to surround the axial length of said helix and eifect the capacitive characteristic of the variable delay line.

6. A variable delay line according to claim 5 wherein said rod is a shorting rod which moves within the axial length of said helix in unison with said inner cylinder in order to short out selected portions of said helix.

7. A variable delay line according to claim 6 including contact means, at least one of said contact means being fixed to one end of said helix and another of said contact means being rotated within the helix to contact peripheral portions thereof.

8. A variable delay line according to claim 7 wherein the efiective delay length of said helix is the portion of said helix surrounded by said dielectric cylinder and not shorted by said rod.

9. A variable delay line according to claim 8 wherein a maximum delay occurs when said dielectric cylinder completely surrounds the axial length of said helix, and a minimum delay occurs when said dielectric cylinder is completely withdrawn from said helix.

10. A variable delay line according to claim 5 including a coaxial cylinder extending from the beginning of said helix to the outer cylinder coupling means to match the transmission line impedance from the point of coupling to the beginning of said helix.

References Cited UNITED STATES PATENTS 2,527,608 10/1950 Willoughby 333-29 X 2,943,276 6/1960 Lovick 33329 HERMAN KARL SAALBACH, Primary Examiner M. NUSSBAUM, Assistant Examiner U.S. Cl. X.R. 333 

